Due to a rapidly increased energy consumption amount and a limited reserve amount of an energy source, problems such as a rapid increase in fuel expenses and exhaustion of the energy source have reached a critical level. Further, together with this, regulations by individual countries on discharging of carbon dioxide have been further tightened. As a countermeasure on these problems, research for natural energy which is capable of being regenerated and is near inexhaustible, such as solar power, wind power, and water power which are clean and do not have a problem of exhaustion of resources, has been more actively made. Particularly, research for using solar power not largely restricted by a location, a position, and the like has come into the significant limelight.
Among methods using solar power as an energy source, recently, a solar cell has been most actively studied. The solar cell is a semiconductor device converting solar light into electricity by using a photovoltaic effect, and a solar cell using an inorganic semiconductor material such as a silicon or compound semiconductor has already come onto the market to be used. Further, recently, an organic solar cell using an organic material such as a polymer resin has been studied.
Light conversion efficiency of the organic solar cell is conversion efficiency of an incident light into electric energy, and refers to efficiency that the incident light excites an organic material in a photoactive layer to form an exciton that is a form where electrons and holes are unstably combined and the electrons and the holes pass through a donor layer and an acceptor layer, respectively, and move to an electrode to be converted into electric energy.
A material for the organic solar cell may be largely divided into a p-type organic semiconductor material that is a donor and an n-type organic semiconductor material that is an acceptor. If the exciton is generated by absorbing light, the p-type organic semiconductor material is the donor which may be divided into the holes and the electrons at a junction with the n-type organic semiconductor material to provide the electrons well. The n-type organic semiconductor material refers to a material which may be reduced into the acceptor to accept the electrons well. Various donor and acceptor materials have been reported, but it is difficult to expect efficiency of 3% or more except for P3HT/PCBM, PCPDTBT/PCBM, or the like.
One of the reasons why the organic solar cell has efficiency which is lower than that of the inorganic solar cell is because the organic solar cell has charge mobility that is much lower than that of the inorganic solar cell. For example, in the case where a thickness of the photoactive layer is increased in order to increase light absorption, due to a reduction in charge mobility and electric field, since re-combination of charges is increased, there is a limit in improving efficiency, and in the case where a large-area cell is manufactured in order to embody a module, due to an increase in resistance of a transparent electrode, an efficiency reduction (cell to module loss) significantly occurs. In order to solve the problems, a novel module structure using an optical spacer, metal nanoparticles, and the like, or minimizing electrode resistance has been proposed.